on day 3 you have to get the 2nd bike you can get it before going to the dock or after that place you get the choice is where you meet the guy who gives you the 2nd bike as far as i'm aware you cant proceed with out the bike and i'm almost sure even though you might think ooo get that bike asap its quicker to go to the harbor first because of what needs done after you are done there becaues of mud and stuff there's only really one path you can really go